The present invention relates to derivation, or recovery, of the timing frequency, or symbol rate, in modulated data signal receivers in which demodulation is effected with the aid of two carriers which are shifted by 90.degree. relative to one another.
The receiver timing frequency can be recovered by various different procedures. For example, by means of a criterion obtained from a received signal, resonators can be excited or control loops can be corrected. A process for stepped synchronization with controllable divider chains is described in German Pat. No. 1,287,609 in which use is made of a phase control loop which is advantageous when dealing with brief losses of clock pulse information since the clock pulse generation continues with a high degree of precision if quartz oscillators are used. However, phase control loops, and even resonators, require for their control a criterion which indicates the desired phase position of the clock pulse. A technique for achieving this is described by R. D. Gitlin and F. J. Hayes in Timing Recovery and Scramblers in Data Transmission, THE BELL SYSTEM TECHNICAL JOURNAL, Vol. 54, No. 3, March 1975, at pp. 569-593. In the method described therein, the timing frequency is acquired from the envelope of the received signal. This process, however, is very expensive as it requires additional filters. Furthermore, when operating with nearly identical carrier and timing frequencies, the beat frequency must be converted in order to obtain the envelope curve. In addition, this process is highly sensitive to noise and distortions.
In the German periodical "Nachrichtentechnische Fachberichte", [Communication Engineering Reports], Vol. 37, 1969, Datenuebertragung [Data Transmission] VDE Verlag, Berlin, at pp. 271-279, in an article entitled "A Four-Phase Modem for 2400 Bits per Second" by J. Haas, there is described a method in which the timing frequency is derived directly from the demodulated signals. This method works well only if applied with differentially coherent demodulation. If coherent demodulation is effected by means of a recovered carrier, the control circuit may become dependent on the timing frequency and the carrier, which will lead to reduced adjustment speeds.